xds: add use_system_root_certs to CertificateValidationContext

[markdroth]

Commit Message: xds: add use_system_root_certs to CertificateValidationContext Additional Description: This allows using system root certs in gRPC. For details, see https://github.com/grpc/proposal/pull/436. Risk Level: Low Testing: N/A Docs Changes: Included in PR Release Notes: N/A Platform Specific Features: N/A

[repokitteh-read-only[bot]]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/). envoyproxy/api-shepherds assignee is @htuch CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

:cat:

Caused by: https://github.com/envoyproxy/envoy/pull/34235 was opened by markdroth.

see: more, trace.

[jmarantz]

@htuch ping